This invention generally relates to a method for making a tooling or a mold for the manufacture of articles having at least one patterned surface, such as, for example, retroreflective sheeting, structured abrasive articles, and adhesive articles for personal care products. Also provided are molds and articles having at least one patterned surface.
Retroreflective sheeting is employed in many applications that enhance the safety of pedestrians and motorists. Many of these applications require the sheeting to have an eye pleasing or cosmetic appearance. One particularly useful type of retroreflective sheeting is cube-corner retroreflective sheeting. These types of retroreflective sheetings typically include a sheet having a generally planar front surface and an array of cube corner reflecting elements protruding from the back surface. The cube corner reflecting elements generally include trihedral structures (i.e., generally having three approximately mutually perpendicular lateral faces meeting in a single corner). In use, the retroreflector is arranged with the front surface disposed generally toward the anticipated location of the intended observers. In this orientation, light incident to the front surface enters the sheet, passes through the body of the sheet to be internally reflected by the faces of the cube corner reflecting elements so as to exit the front surface in a direction substantially toward the light source, i.e., retroreflection.
The manufacture of retroreflective cube corner element arrays is typically accomplished by employing molds primarily made by known techniques, including pin bundling and direct machining. Molds manufactured by pin bundling are made by assembling together individual pins which each have an end portion shaped with features of a cube corner reflective element. The direct machining technique, also known as ruling, involves cutting away portions of a substrate to create a pattern of grooves that intersect to form structures including cube corner elements. This grooved substrate is typically used as a master from which a series of impressions, replicas, or molds may be formed. These are typically then used as molds for retroreflective sheeting. An example of direct machining is described in U.S. Pat. No. 4,588,258 (Hoopman).
Once the mold is made, retroreflective sheetings are then typically made either by thermally embossing a plastic sheet with the grooved substrate to form a molded surface or by subsequently depositing a crosslinkable, partially polymerized resin, on a mold to be replicated which is then typically exposed to radiation, e.g., actinic light or heat, to solidify the resin. An example of such replication is described in U.S. Pat. No. 3,689,346 (Rowland).
Such manufacturing processes are typically continuous processes. For continuous manufacturing of retroreflective sheeting, a tool is generally formed from a flat originally ruled substrate, or a replica thereof, into a cylinder with one or more welding lines across the width of the sleeve. The resin composition flowing into the weld line tends to stick to the molding surface and cause objectionable seam lines and defects in the resulting sheeting. Moreover, in the step of bonding an overlay film to the array of cube corner elements, defects tend to result when the weld line aligns with an embossing protrusion on an embossing roll.
The efficiency and appearance of retroreflective sheeting can be affected by thermal or mechanical stresses, effects of resin shrinkage, removal from the mold, and the shape of the mold itself. For example, in a majority of retroreflective sheeting, seam lines can be observed across the width of the retroreflective sheeting. Because these seam lines reduce the cosmetics of the sheeting and, in some instances, impair the overall retroreflectivity of the sheeting, attempts have been made to eliminate them. For example, U.S. Pat. Nos. 5,643,400 and 5,558,740 (both to Bernard et al.) describe an apparatus and a method, respectively, for producing retroreflective sheeting, wherein at least two mold surfaces are used to generate two prism arrays which are overlapped at a leading and/or a trailing edge of each array.
What is yet needed is a method for making a tool having at least one patterned surface with a sufficiently strong weld that is capable of producing a cosmetically pleasing narrow seam line on an article having at least one patterned surface, such as retroreflective sheeting.
As used herein, xe2x80x9ctoolingxe2x80x9d or xe2x80x9ctoolxe2x80x9d refers to a substrate having at least one patterned surface that forms an original template from which other articles can be replicated, such as a mold or an article, such as retroreflective sheeting, an abrasive article, and the like. Typically, the tooling contains multiple patterned tiles that are joined together forming lay-up lines between the individual tiles. The tooling may include more than one tooling segment, which could be utilized as an original template by itself.
As used herein, xe2x80x9cmoldxe2x80x9d refers to a structure formed by the tooling. It is the mold that typically is utilized in further replication processes in producing articles such as retroreflective sheeting, an abrasive article, and the like.
The present invention provides a method of making a tooling, including providing a substantially planar tooling having a first end and a second end opposing one another, a patterned side, and a back side opposite the patterned side; placing the opposing ends together to form a substantially cylindrical shape forming a lumen therein, wherein the back side faces the lumen; and welding the ends together from the lumen such that at least the opposing ends of the back side are joined. In one embodiment, the substantially cylindrical shape has a substantially circular cross section.
Preferably, welding the ends together includes welding the ends together from the lumen with less than 100% penetration of a resulting weld. The opposing ends are preferably held together during the welding process using a fastener selected from the group of a mechanical clamp, a magnetic plate, or application of a vacuum. In accordance with the present invention, welding the opposing ends preferably produces a joining line having a width of about 0.0025 mm to about 0.2 mm on the patterned side. The tooling may also include more than one tooling segment such that the tooling comprises more than one joining line having a width of about 0.0025 mm to about 0.2 mm on the patterned side.
In one embodiment, the tooling includes a metal. Preferably, the metal is selected from the group consisting of aluminum, brass, copper, nickel, and combinations thereof. If desired, other materials and/or metals may be used.
Welding the opposing ends in accordance with the present invention preferably includes exposing the back side of the tooling to a laser selected from the group consisting of a carbon dioxide laser, a ruby laser, an Nd:glass laser, and an Nd:YAG laser. The tooling is preferably exposed to a laser at a feed rate of about 2.5 cm/minute to about 1600 cm/minute. The tooling is preferably exposed to a laser at a pulse rate of about 5 pulses per second to about 100 pulses per second. The tooling is preferably exposed to a laser at a power per pulse of about 20 joules or less per pulse.
A method in accordance with the present invention may also include placing a heat sink adjacent to the patterned side after placing the opposing ends together.
A joint formed in accordance with the present invention may result from one of the configurations selected from the group consisting of a butt joint, a wedge joint, an overlapping joint, or a raised ridge joint.
Another aspect of the present invention is a mold produced by the tooling made by the method as described above, wherein the mold comprises a joining line having a width of about 0.0025 mm to about 0.2 mm on the patterned side.
A further aspect of the present invention provides an article including at least one patterned surface produced using a mold described above, the at least one patterned surface having a seam of substantially the same width as the joining line of the mold.
Yet another aspect of the present invention provides a microstructured composite sheeting including a three dimensional array of cured microstructure elements formed from a polymeric material, wherein any seam present in the array has a width of about 0.0025 mm to about 0.2 mm on the patterned side.
Another aspect of the present invention provides a mold for making an article having a patterned surface, the mold comprising a patterned surface outer surface, an inner surface, and a joining line having a weld penetration of less than about 100% of a tooling thickness.
The present invention addresses the problem of reducing seam appearance at the tooling, or substrate, stage so that the process of producing patterned surface articles (sometimes referred to herein as xe2x80x9creplicatexe2x80x9d) remains relatively simple, as opposed to addressing the problem at the replication stage, as described in U.S. Pat. Nos. 5,643,400 and 5,558,740 (both to Bernard et al.).